The present invention relates to a magnetostatic wave microwave device utilizing ferrimagnetic resonance and comprising a magnetostatic wave element and a means for exciting magnetostatic waves, which is used in many microwave components and in particular is used in oscillators.
Recently, as a magnetostatic wave microwave devices for use in a filter, an oscillator, etc. which utilizes ferrimagnetic resonance of a ferrimagnetic thin film, those having a magnetostatic wave element which is produced by forming a ferrimagnetic YIG (yttrium-iron-garnet) thin film on a non-magnetic, single crystalline substrate of GGG (gadolinium-gallium-garnet) by liquid phase epitaxial growth (LPE), etching it by a photolithographic technique and machining the etched film into a desired shape such as circle, rectangle, etc. has been proposed. These magnetostatic wave microwave devices have advantage because they can be made into a microwave integrated circuit using a microstrip line, etc. as a transmission line, and they can be easily connected with other microwave integrated circuits by hybrid junction.
In addition, since a magnetostatic wave element utilizing YIG film can be produced by LPE and machining, it has a high productivity as compared with a conventional magnetostatic wave element utilizing YIG spheres.
However, the performance of a magnetostatic wave microwave device utilizing ferrimagnetic resonance of YIG film is significantly dependent on geometric shape of the YIG film. In order to obtain a large impedance, a YIG film having a large surface area and large thickness is required. However, these large surface area and thickness are disadvantageous from a view point of miniaturizing microwave circuits and maintaining a ferrimagnetic resonance of a large Q-value.
Generally, a circuit, as shown in FIG. 15, in which an LC parallel resonator of concentrated-constant type is connected at a distance d from an input terminal T is an ideal equivalent circuit for magnetostatic wave microwave devices. In order to obtain a large impedance change, the length of the magnetostatic wave element along the propagation direction of microwave has been made longer in the prior art. In this method, however, the length L of the magnetostatic wave element becomes comparable to the wavelength of microwave so that the equivalent circuit in this case is expressed by a distributed constant circuit, as shown in FIG. 16, in which an indefinite number of LC resonators are connected along the direction of L. This corresponds to observing simultaneously a plurality of resonators excited by microwaves of different phases, resulting in failure to obtain a high Q-value of the magnetostatic wave element.
On the other hand, when the length L, width W and thickness t of the magnetostatic wave element are simultaneously made smaller in order to obtain a high Q-value, although a higher Q-value can be obtained, the magnetostatic wave element absorbs very little energy from a microwave. This means that the impedance change at resonance is significantly reduced.
FIG. 12 is a schematic view showing a conventional magnetostatic wave microwave device utilizing a rectangular YIG film. A rectangular, thin magnetostatic wave element 1 is disposed between a ground conductor 3 and a microstrip line 2 which is shorted at the end thereof. An external magnetic field Hext for causing ferrimagnetic resonance is applied perpendicularly to the surface of the magnetostatic wave element 1. Microwaves propagate along the direction of an arrow 4a and are reflected in the direction of an arrow 4b. The reference character Wi means the width of the microstrip and the reference character d means the distance between the edge of the microstrip line 2 and the nearest edge of the magnetostatic wave element 1. In the conventional device, the length L and the width W of the magnetostatic wave element 1 are nearly the same, i.e., the upper surface of the magnetostatic wave element 1 is nearly square. In the prior art, in order to obtain a larger impedance change at resonance measured from the input portion of the microstrip line, L, W and t of the magnetostatic wave element 1 have been simultaneously made longer. However, as mentioned above, increased L, W and t result in reduction of the Q-value of the magnetostatic wave element 1. In contrast, decreased L, W and t lead to reduction of the impedance change at resonance.
FIG. 13 is a schematic view showing a conventional magnetostatic wave microwave device utilizing a circular YIG film. In the conventional magnetostatic wave microwave device, when the outer diameter D and the thickness t of the magnetostatic wave element 1 are made larger in order to obtain a large impedance change at resonance, the Q-value of the magnetostatic wave element 1 is reduced. When D and t are made smaller for obtaining a high Q-value, the impedance change at resonance is reduced.
FIG. 14 is a schematic view showing a conventional magnetostatic wave microwave device having an electrode finger structure 7. This magnetostatic wave microwave device also has the same disadvantages as those in the magnetostatic wave microwave device shown in FIGS. 12 and 13. Namely, when the length L and the thickness t of the magnetostatic wave element 1 are made larger in order to obtain a large impedance change at resonance, the Q-value of the magnetostatic wave element 1 is reduced. On the other hand, when the length L and the thickness t are made smaller for obtaining a high Q-value, the impedance change at resonance is reduced.
FIG. 17 is a schematic illustration showing a magnetostatic wave microwave device in which a right end-shorted microstrip line 2 is disposed with a magnetostatic wave element 1 at a short portion. A wave I with hatched lines shows a standing wave of a microwave current. The microwave current has the maximum magnitude at the short portion. The reference numeral 3 is a ground conductor.
In this device, a larger impedance change of the magnetostatic wave element 1 at resonance can be attained by elongating the sides of the rectangular magnetostatic wave element 1 along the propagation direction of microwave (FIG. 18). The disadvantage in this method has been described above by referring to FIG. 16.
Alternative method for attaining a larger impedance change is to increase the thickness of the magnetostatic wave element 1 (FIG. 19). However, the shape of the magnetostatic wave element 1 is unfavorably deformed from flat spheroid which is regarded as an ideal shape for the magnetostatic wave element 1, thereby resulting in a significantly reduced Q-value of resonance characteristics.
Still another method for attaining a larger impedance change is to make the width (perpendicular to the propagation direction of microwave) of the magnetostatic wave element 1 longer than the length (along the propagation direction of microwave) of the magnetostatic wave element 1 (FIG. 20). However, a greater part of the magnetostatic wave element 1 comes to deviate from the center line of the microstrip line 2 with increasing width so that the coupling between microwave and the magnetostatic wave element 1 is not necessarily sufficient.